Very high heat dissipations occur in high power electronic devices such as high-energy laser and the high power microwave devices. Surface heat fluxes for laser diodes are approximately 100 to 500 w/cm2. Microwave interaction and collector cavity heat fluxes can reach 1000 to 2000 w/cm2 respectively. High power electronics such as converters, inverters and motor drives typically have devices that generate heat fluxes of 10 to 40 w/cm2 at the device level and even higher at the die level. High power CPU packages for high performance computers will dissipate as much as 100 w/cm2 over a footprint of a few square centimeters. All of these devices must operate within acceptable temperature ranges regardless of their heat dissipation. Such devices also demand good surface isothermality for optimum device performance. Where the ultimate heat sink is at a temperature level that is too high for direct or cascaded loop cooling, rejection of device waste heat to the heat sink requires thermal pumping to a higher temperature level. An efficient refrigeration system is a vapour cycle wherein an evaporator absorbs heat at a lower temperature and a loop condenser rejects it at a higher temperature. Vapour cycle systems offer two advantages in thermal control at the evaporator. First, pressure level may control temperature due to the pressure-temperature relationship of the saturated vapour. Second, vapour cycle systems exhibit better heat source isothermality than single-phase systems because the coolant changes temperature with heat addition in such systems, such as with the compact high intensity cooler (CHIC) described in U.S. Pat. No. 6,167,952 to Downing. Given these and other advantages of two-phase heat absorption, a high performance evaporator must be capable of accepting high heat fluxes and providing nearly constant temperature heat rejection over the device footprint. Additionally, the thermal resistance of the device should be small, thereby reducing the required lift of the refrigeration system. The evaporator should be capable of evaporating the coolant to high outlet qualities without high pressure drops that would penalise the cycle. Vapour specific volumes are in the range of 140 to 1000 times larger than their liquids. To manage flow velocities and thereby pressure losses during the large changes in volumetric flow a flow structure with and expanding flow area is required. Flow velocities should remain large enough to maintain shear control, however. These conditions serve to maintain annular flow and wet wall conditions capable of withstanding high heat fluxes to high outlet qualities.
Some high energy systems that have short duty cycles make expendable coolants an attractive solution to energy management. In this case, stored liquid coolant may evaporate in the evaporator and then vent in an open cycle arrangement. The desired features of an evaporator for this open cycle system are identical to the closed cycle heat absorber.